In recent years, there has been a significant development in the correction of dentofacial deformities. In particular, many devices and techniques have been developed for correcting deformities of the mandible. The Haas appliance is an example of one well-known device that provides lateral maxillary expansions. The Haas device is a mechanism constructed of two parts that is expandable by way of a threaded screw arrangement. The four arms attached to the device are anchored to the tooth structures to provide an expansion pressure between the teeth. The posterior portion of the mandible is completely severed, preferably between the alveolar tooth sockets of the bone. The Haas device, located between the left molars and right molars of the mandible can then be expanded to separate the mandible and provide lateral expansion. Because the Haas device is mounted to the tooth structures, it is known as a "tooth borne" device. The disadvantage with a tooth borne expansion device is that the teeth are not rigidly anchored to the mandible, and thus a proportionate symmetric expansion force cannot be achieved between the severed portions of the mandible. Mid-line, lateral and rapid mandibular expansion techniques utilizing the Haas appliance are disclosed in the text entitled Modern Practice in Orthognathic and Reconstructive Surgery, 1992, by Dr. William H. Bell, pp. 2383-2394.
In addition to the lateral expansion techniques, dentofacial deformities also exist in which the frontal portion of the mandible is retruded with respect to the corresponding maxilla. This is commonly called mandibular retrognasthism. There currently exist difference techniques for correcting this mandibular deformity by extension thereof in a forward direction. The procedure is well known as mandibular advancement, and includes sectioning the mandible on opposite sides, preferably between the ramus and the posterior molars. The cut is generally vertical, but laterally angled through the mandible. Then, the mandible is repositioned anteriorly the desired amount, and the segments of the bone are reattached by pins, screws or similar fasteners. Generally, the amount by which the mandible can be moved forward with this technique is limited to about 10-15 mm. The disadvantage with this procedure is that the amount of forward movement of the mandible with respect to the ramus must be accomplished at one time during the operation. The amount of forward movement is limited due to the inability of the muscle and surrounding tissues to stretch without relapsing. Sometimes, the amount of forward movement of the mandible for proper correction is significantly more than can be accomplished with this technique. Relapsing may also compromise the result. Additionally, mandibular advancement may not be feasible in children who frequently have significant psychological problems.
In order to overcome the problems of relapse and the foregoing problems, and allow treatment at an early age in an ambulatory setting, osteodistraction appliances have been developed that can mechanically reposition the mandible forward (when severed from the ramus) incrementally over a period of time. One such type of device is shown in FIG. 1. The lengthening appliance 10 is essentially a two-piece device that is extendible by way of threaded parts. A first part 12 is separable from a second part 14 by way of a threaded screw 16 that is engaged within a respective threaded bore (not shown) formed through each such part. An apertured collar 18 is fixed to the screw 16 and is rotatable by way of the radial holes 20 to provide adjustability between the parts 12 and 14. In addition, the first part 12 has fixed thereto a pair of rod-shaped guide rails, one shown as reference numeral 22, that are slidable through corresponding bores 23 formed longitudinally through the other part 14. In this manner, the one part 12 does not rotate with respect to the other part 14 when the screw 16 is rotated, or during regular movements of the mandible during chewing motions. Attached to the one part 12 is a pair of eyelets, one shown as reference character 24. Similar eyelets 26 are fastened to the other part 14.
In using the extension appliance 10, the mandible is severed by a saw or other suitable instrument. The one part 12 of the lengthening device is fastened to one portion of the severed mandible by the use of surgical screws that pass through the eyelets 24 and are secured to the bone. In like manner, the eyelets 26 of the other part 14 are fastened to the other portion of the mandible by another set of surgical screws. A similar extension appliance 10 is fastened in a similar manner to the opposite side of the severed mandible. Then the collar 18 is rotated a sufficient number of revolutions to reposition the mandible a predefined amount. Preferably, the appliances 10 are both adjusted the same amount. The rotation of the screw 16 is effective to separate the one part 12 of the appliance from the other part 14, thereby separating the severed portion of the mandible. At daily intervals, the collar 18 can again be rotated a prescribed amount to thereby incrementally lengthen the mandible. Over a period of time, such as three weeks, the mandible can be lengthened by as much as 25-30 mm.
While the extension appliance 10 shown in FIG. 1 is effective to incrementally lengthen the mandible, the inherent shortcoming is that the appliance must be held in place against the severed mandible as the screws are driven through the eyelets into the bone. Another disadvantage is that because the eyelets are rigidly fixed to the body of the appliance, there may be difficulty in placing the screws in the bone and avoid the developing teeth. To remove or replace the appliance, the surgical screws must be removed. This is an advantage of the appliance, procedure and can be carried out under local anesthetic in an ambulatory setting.
FIG. 2 shows an improved bone distraction device, as compared to that shown in FIG. 1. The bone distraction device 30 has a similar extension mechanism, but includes elongated wire arms, one shown as reference numeral 32. The one part 12 includes a pair of arms 32, each fixed thereto, and extending at an angle. Moreover, the end of each arm has an integral forked end 34. Forked end 34 includes a slot 36 for engaging with a surgical screw fastened or otherwise anchored to the mandible. The other part 14 includes a similar angled arm 32 with a forked end 34. However, the other part 14 includes a circular band 38 fastened to the part 14 by way of a wire arm 40, or the like.
FIG. 3 illustrates the bone distraction device 30 fastened to the base portion of the mandibular ramus 42 by a pair of surgical screws 44. On the other side of the cut bone 46, the device 30 is anchored by way of a screw 48, as well as the band 38 that is anchored to a tooth. The appliance 12 is anchored on the other side of the bone cut 46 by way of the two forked ends 34 anchored to the respective surgical screws 44. Such a device is known as a "tooth borne" bone distraction device.
The device 30 shown in FIGS. 2 and 3 has several inherent shortcomings. First, the band 38, being anchored to the tooth, is often problematic. Another shortcoming and often a serious problem, is that the forked ends 34 require that the screws 44 and 48 be precisely positioned and fastened to the severed parts of the mandible so as to accommodate the bone distraction device 30 therebetween. As can be appreciated, if one of the surgical screws is inadvertently fastened to the bone too far from the device, there may be a nonsymmetrical force applied across the cut in the bone. The tendency is thus to separate the bone parts in an unequal manner. Another disadvantage of the tooth borne appliance is that the involved tooth is not rigidly fixed to the mandible, but is somewhat movable. As such, the extension force on the tooth may not result in a symmetrical and proportionate separation of the bone, as some of the extension force is absorbed by tooth movement rather than bone movement.
As can be appreciated from the foregoing, a need exists for an improved bone distraction device and method of installation thereof, that overcomes the shortcomings and disadvantages of the prior art devices. A specific need exists for a bone distraction device that does not require specific spacing of the anchor screws in the mandible. Another need exists for a fully bone borne distraction device that is anchored solely to the bone to provide uniform, proportionate and symmetric separation of the severed bone parts. Another need exists for a device that is less critical of the precise location at which the surgical screws are driven into the bone. Yet another need exists for an expansion device constructed such that the anchor ends are adjustable with respect to the body of the device. A further need exists for a technique in which the bone can be sectioned at several locations, and multiple expansion devices employed to reposition the bone in multiple dimensions.
The principal indication for widening the mandible is absolute transverse mandibular deficiency. An excessively narrow and tapered arch form, dental crowding, tipped teeth and congenitally missing teeth are additional reasons for use of surgery which is intended to normalize basal bone position and facilitate non-extraction orthodontic treatment. Incomplete telescopic bite in certain congenital problems (Pierre Robin, Treacher Collins, Hemifacial Microsomia), and combined maxillo-mandibular transverse deficiency may be additional indications. Also, patients with mandibular transverse deficiency whose crowded teeth have been treated by extraction orthodontic therapy, may be additional indications. Many of these individuals may benefit from surgically assisted rapid mandibular expansion and orthodontic treatment.
Transverse mandibular deficiency is commonly managed by orthodontic mechanics which might include extraction and dental compensations. The result of this approach may be unstable owing to tipping of the teeth and bending of the alveolar bone. Proffit and Ackerman have reported a high risk of dental relapse when compensating orthodontic therapy has been performed to increase the inter-canine width in the presence of a primary transverse bone deficiency. Indeed, Proffit and White have documented the limitations and easy violation of the transverse envelope.
When a skeletal or dento-alveolar deformity is so severe that the magnitude of the problem lies outside the envelope of possible correction by orthodontics alone, surgical orthodontic treatment is indicated. Osteodistraction techniques may be the key to optimal nonextraction management of many malocclusions thus maintaining and increasing the functional occlusal table. Attention to transverse deficiency is vital in planning treatment for patients who require an increase in the lateral dimensions of the mandible or maxilla. The transverse envelope of discrepancy for mandibular alterations can be addressed by symphyseal osteotomy and gradual osteodistraction with the bone supported distraction device of the present invention.
When distraction appliances are not fixed to the bone (tooth supported appliance) there is typically disproportionate movement of the bone associated with expansion of the device. With the devices known in the prior art, tooth movement and tipping, additional to bone extraction, was frequently seen in patients. The osteodistraction appliances of the present invention produce proportionate movement of the teeth and bones. Additionally, the tooth borne appliance of the invention permits individualization of the osteotomy design and osteodistraction. With the new appliance design, not only can the mandible be lengthened and widened, but the maxilla can also be lengthened and widened. Indeed, a three-dimensional repositioning of the maxilla and mandible can be achieved in a more predictable manner and there is a proportionate movement of the mandible and maxilla transversely, vertically and anterior-posterior.